Ok yeah it does sound like a voice coil problem then.. The warranty should cover it as long as you bought from an authorized dealer.. I'd definitely try giving them a call too see if you can talk to a live person.. When dealing with infinity i had absolutely no luck until i talked to a person.. And all i really wanted out of the tech support was to see if there was a reason my speaker blew, and how i could prevent it, when he told me i could ship back the cone and voice coil and get brand new ones straight from the factory, even though i bought from an unauthorized dealer.. he said only a one time replacement though.. I dont know if pioneer would be that helpfull but if you did buy from an authorized dealer they should definitely help you.. Also, if you bought if from a retailer in town take it back to them, lots of times they will warrant it and give you a new one and you wont even have to pay shipping. It doesnt hurt to try..
To fix the sub would be very difficult if not impossible. it wouldnt be worth your time if you have voice coil problems.. it wouldnt be as good as new that way either.